x86/tsc: Remove CPUID "frequency" leaf magic numbers.

All the code that reads the CPUID frequency information leaf hard-codes
a magic number.  Give it a symbolic name and use it.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/all/20241213205036.4397658F%40davehans-spike.ostc.intel.com